1. Field of the Invention
The present invention relates generally to electronic packaging technology and, more particularly, to semiconductor packages having improved ball land structures.
2. Description of the Related Art
A recent high-integration technology for integrated circuit (IC) makes it possible to form and connect much more circuits in and on a semiconductor chip at a given chip size. Thus, a package, which contains the semiconductor chip therein, also needs more input/output (I/O) terminals communicating with external circuitry such as a motherboard.
In order to meet such a need, a ball grid array (BGA) package has been developed wherein the I/O terminals are distributed over a package surface, rather than just located peripherally at one or more package edges as in a conventional lead frame package. Typically, an array of solder balls provides the surface-distributed I/O terminals for the BGA package. Such distribution of terminal locations radically allows a reduction in a board surface required for the BGA package when the package is mounted on the motherboard.
In the conventional BGA package, the solder balls are formed on a lower surface of a substrate, and the semiconductor chip is attached on an upper surface of the substrate. The chip is electrically connected to the substrate through wires and encapsulated with resin encapsulant. Hereinafter, the lower surface of the substrate will be referred to as a ball-mounting surface. Further, a region of the ball-mounting surface on which the solder ball is formed will be referred to as a solder ball land.
FIG. 1A schematically shows, in a plan view, one type of the solder ball land of a conventional BGA package. This type is generally known as a solder mask defined (SMD) type.
As shown in FIG. 1A, a solder ball land 10 and a wiring pattern 14 connected thereto are formed on the ball-mounting surface of the substrate (not shown). The solder ball land 10 is generally shaped like a circle and made of copper. Further, the solder ball land 10 is preferably plated with nickel and gold for enhancing a reliable attachment of a solder ball (not shown). A solder mask 16 covers the overall ball-mounting surface, together with the wiring pattern 14 and a peripheral portion 10a of the solder ball land 10, but exposes a central portion 10b of the solder ball land 10 through an opening 16a. 
FIG. 1B schematically shows, in a plan view, another type of the solder ball land of a conventional BGA package. This type is generally known as a non-solder mask defined (NSMD) type.
As shown in FIG. 1B, a solder ball land 20 and a wiring pattern 24 connected thereto are formed on the ball-mounting surface 1 of the substrate. A solder mask 26 covers the ball-mounting surface 1 and a main portion 24a of the wiring pattern 24, but exposes an end portion 24b of the wiring pattern 24, the entire solder ball land 20, and parts of the ball-mounting surface 1 around the solder ball land 20, through an opening 26a. 
As widely known in the art, the above-described conventional solder ball lands 10 and 20 have the following drawbacks.
In the case of SMD type, during a solder joint reliability test that evaluates a joining strength of the solder ball after the BGA package is mounted on the motherboard, the solder ball is often cracked or detached from the solder ball land. Further, in the case of NSMD type, after the BGA package is mounted on the motherboard, the wiring pattern is disconnected from the solder ball land, or the solder ball land is peeled from the ball-mounting surface of the substrate.
These issues related to the solder ball lands often arise in another types of packages, using solder ball connections, such as wafer-level packages, flip-chip packages, and so on.